Metallic rolling elements for aerospace bearing applications are typically made of specific materials and according to an exacting set of process steps. For example, they are often made of AISI 52100 or M50 steels and are subject to the prescriptive steps of (a) thermo and/or mechanical processing to control the shape and size of the rolling elements and rings; (b) heat treatment comprised of austenitizing, quenching, cold treating, and tempering to develop the desired hardness, compressive stress, fracture toughness and surface residual stress profile; (c) abrasive machining according to specification requirements; and (d) non-destructive inspection for quality assurance. The above prescriptive practices have evolved over the past many decades and have met the requirements of military and commercial main shaft bearings.
However, to increase the power output and performance of current engines, the aerospace propulsion industry has progressively increased the performance requirements for aerospace bearings, which includes rolling contact fatigue life and durability, higher temperature operation, higher speed and lighter weight. Consequently, the industry has now approached the material capability limits of conventionally produced rolling elements made of AISI 52100, M50 steels and the like including martensitic stainless steels. More recent engine designs have increased the engine loads to a point where they put far more applied stresses on the bearing rolling elements and rings.
In most rolling element bearings, the rolling element is a ball, which rolls between inner and outer rings called races. The balls are separated by pockets in a cage ring, which keeps them evenly spaced around the races. When running in operation under load, the metal of the bearing rolling elements is subjected to stresses of enormous intensity, which will cause cyclic flexure, compressive and secondary tensile stresses, and sliding of different contacting surfaces. It may cause deformation under extreme conditions. Over the course of the life of a bearing, the alternating rolling contact fatigue stresses may happen at a given stress volume in the rolling element many millions of times. In addition, because of the very small contact area (compared to cylindrical or conical rolling elements) between the rolling element and the races, the localized maximum stresses are especially severe. For this reason, the requirements of strength and rolling contact fatigue resistance properties are most demanding for bearing rolling elements and rings.
The total fatigue stress a bearing rolling element/ring experiences is equal to the surface/sub-surface residual stress plus the operational stress caused by the applied load. One way to increase the load capacity for the part is to decrease its residual stress. The residual stress is defined as the stress which remains inside a component or structure after the applied forces have been removed. On the one hand, compressive residual stress localized at the surface and sub-surface region of the component is beneficial as it off-sets the bearing operational contact stress thereby increasing the engineering margin of the rolling element and/or ring capability to operating stresses. On the other hand, tensile residual stress in the surface of the component is detrimental since it is additive to the operational stress, thereby decreasing the component's fatigue capacity and life.
Current manufacturing processes rely on the afore-mentioned heat treatment to provide the mechanical property requirements needed in bearing rolling elements and rings. The principle of bearing steel heat treatment is to produce a tempered martensitic structure to achieve the required balance of hardness, rolling contact fatigue resistance, fracture toughness and dimensional stability. While effective to a point, it simply does not meet the increased property requirements of high performance bearings into the future.
To better answer the challenges raised by the aerospace industry to produce increased capability bearing rolling elements, it is therefore desirable to increase the compressive residual stress in the surface & sub-surface region of the rolling elements and make the resulting surface capable of handling the increasing loads and speeds. Further, similar treatment to the surface of the raceway in a bearing ring is desirable as well.